Coated abrasive materials are usually produced in large rolls from which the desired commercial product is cut by an automated process. One of the most useful forms of coated abrasive is in the form of a belt. For many years, endless abrasive belts and cones have been made by splicing the ends of lengths of coated abrasive sheet material. Because of the conventional manufacturing process, each such belt must be produced from a strip of suitable dimensions with the ends of the strip joined together to make a continuous loop.
Two types of splices are common, and this may be done by profiling each end such that, when overlapped and joined, the belt has a substantially uniform cross-sectional thickness across the joined area. This is known as a xe2x80x9clapxe2x80x9d joint in which the ends of the length are beveled by removing abrasive granules from one end and part of the backside from the other end, and the beveled ends are overlapped and joined adhesively.
Alternatively, both ends of the strip to be joined to form the belt can be hollowed on the non-abrasive-bearing surface to accommodate a joining strip that is bonded to each of the hollowed ends. This is known as a xe2x80x9cbuttxe2x80x9d joint. In the so-called xe2x80x9cbuttxe2x80x9d splice, the backside at each end of a length of coated abrasive sheet material is scuffed to form a hollowed out space which is filled with an adhesive along with a strong, thin, tear-resistant splicing tape. Often, each type of splice is formed in a heated splicing press to be of substantially the same thickness as the remainder of the belt or cone.
The material of the backing on which the abrasive material is carried can be a woven or non-woven fabric, a plastic film or sheet or a paper of suitable durability. Most backings of endless abrasive belts and cones are cloth, paper, polymeric film, or laminates thereof. Cloth backings provide durable backings but are expensive and, to have suitably smooth surfaces, can require a series of coating treatments that can make cloth backings even more expensive. Paper and laminates of polymeric film and paper afford intermediate durability and are often used where cloth would be too expensive. For light duty, backings can be polymeric film, the backside of which usually has a resinous coating that is porous.
To form the joint, it is conventional to prepare the ends by treating them to ensure that the adhesive penetrates and bonds firmly to the substrate. This is particularly important if the backing has been given a substantial backfill treatment. Where the backing is a fabric, the ends can be sandblasted to open up the exposed fiber ends and allow penetration of the adhesive into the material of the backing.
The adhesive used must meet demanding standards to ensure that the belt joint remains intact throughout the useful life of the belt. This is important since a sudden joint failure while the belt is in use could be very dangerous to both the machine and the operator.
Two component polyurethane adhesive systems have been generally used as the adhesives in joining coated abrasive belts. The adhesive systems, which produce tough and flexible bonds, generally consist of high molecular weight urethane and a multi-functional isocyanate crosslinking agent.
However, due to the high reactivity of isocyanate with materials such as amine, alcohol, and water, the two component polyurethane systems suffers at least two drawbacks in manufacturing environments including limited pot life and sensitivity to the relative humidity of the ambient air. FIG. 1 shows a graph which displays the variation of the quality of a belt joint of a typical prior art joint system with the age of the system. The graph shows a pot life for that system of about four hours.
A blocked isocyanate urethane system is based on reversibility of the addition reactions of isocyanates at elevated temperatures. It is relatively stable at ambient temperature, but when heated the isocyanate undergoes a de-blocking reaction, i.e., the release of the blocking agent, to the isocyanate before reaction of the isocyanate with a nucleophile, for example, a hydroxyl.
During the formation of the polyurethane adhesive, a polymerization reaction occurs between an isocyanate (xe2x80x94NCO) group in a isocyanate component and a group in a hydroxyl component containing an active hydrogen group, such as a hydroxyl group in a polyol. The isocyanate component contain two or more isocyanate groups and the hydroxyl component contains two or more hydroxyl groups.
The isocyanate group of the isocyanate group is blocked so that it cannot react with the active hydrogen groups of the hydroxyl component, such as a polyol. The blocking group is removed from the isocyanate group by heating to an elevated temperature, thereby allowing the exposed isocyanate group to react with a hydroxyl group and form the polyurethane. The polyurethane formed by this method is considered a blocked isocyanate urethane system.
It has been discovered that blocked isocyanate urethane systems greatly improve pot life and minimize environmental concerns. In one embodiment, an adhesive for use in coated abrasive belt joints includes a blocked isocyanate, urethane system. The blocking agent can include phenols, oximes, alcohols, caprolactam, and diethyl malonate. In one embodiment, the adhesive is crosslinked with an amine. In another embodiment, the adhesive is crosslinked with an alcohol.
A coated abrasive belt is formed by attaching a plurality of abrasive particles to a first surface of a backing layer. A size coat is deposited over the abrasive particles to form a coated abrasive. The coated abrasive is cut to form a strip having a first end and a second end. The ends of the strip are joined with an adhesive including a blocked isocyanate, urethane system.